In the field of power conversion, the switched-mode power supply (SMPS) is a standard choice. Different implementations of SMPSs are known comprising two controllable switches along with an inductor as a synchronous converter or one controllable switch along with an inductor and a diode or capacitor as an asynchronous converter. In the following discussion, the step-down, or buck, converter will be used as the example SMPS topology; however, the concepts may also be applied to boost, buck-boost, Class E, etc. converters.
The output voltage of a SMPS is controlled by switching the controllable switch(es) on and off, wherein usually a high frequency control loop and a low frequency control loop are used for controlling the controllable switch(es). Using an asynchronous SMPS with one controllable switch as an example, the output voltage may be controlled on the basis of the switching frequency or on the basis of the duty cycle of the controllable switch, wherein a duty cycle control can be achieved by comparing a ramp signal having a fixed frequency with a reference signal and the switching time is determined when the ramp signal reaches the reference signal. In this case, the low-frequency control loop provides the reference signal e.g. on the basis of the input current, the output current, the output voltage or the output power, and the high-frequency control loop controls the switching based on the ramp signal and the reference signal. Usually the controllable switch is switched on when the ramp signal is lower than the reference signal and the controllable switch is switched off when the ramp signal is higher than the reference signal. The output power or output current of the converter can be controlled by setting the reference signal to a higher value or a lower value by means of the low-frequency control loop.
In order to minimize the switching losses of the converter unit, the switching time has to be synchronized to the voltage across the controllable switch. In asynchronous SMPSs, the controllable switch is switched on when the voltage across the controllable switch is zero (zero voltage switching) or reaches a local minimum (valley switching). This serves to minimize the switching losses of the controllable switch.
Existing state of the art controllers are configured to switch the controllable switch on when the current in the inductor is zero or when a minimum value is detected. In these controllers, it is common to utilize an on-time control determined by the low-frequency control loop in combination with an oscillator circuitry such as a ramp signal in combination with a reference signal.
In an alternative solution, the controllable switch is switched off when the inductor current reaches a peak threshold and the controllable switch is switched on when the inductor current crosses zero. However, due to switching delays and reverse recovery times of the diodes, this may yield a suboptimal switching time; hence, switching losses are not minimized with such a scheme.
From EP 2 528 216 A1, a self-oscillating buck converter is known using a threshold control on the basis of the inductor current and the switch voltage. However, the detection of the on transition of the controllable switch is difficult for AC to DC applications when a rectifier unit is connected to the input terminals. This is due to either zero-voltage- or valley-switching being required at different points in the ac cycle, which is not easily handled via a fixed voltage threshold. As a result, the switching losses are not minimized in such a case.
US2012286752 (D1) discloses a control circuit for a switching regulator. The circuit comprises an inductor, a controllable switch and a current detector. The detected inductor current is used to control the switch. The control circuit also includes a slew rate control that controls an electric current value of at least one of a high-side variable current source and a low-side variable current source, which slew rate control defines the rate of change of the switch voltage signal provided to the switch.
US488882 (D2) discloses a synchronization circuit for a resonant flyback high voltage supply. The circuit is provided with a negative slope detection circuitry. The basic function of the negative slope detection circuitry is to disable a switch during the falling slope of the flyback waveform until the dv/dt of the negative slope is equal to zero.